Due to rapidly widespread use of mobile phones or other devices in recent years, there is an increasing demand for a memory with characteristics including nonvolatility, large storage capacity, low-voltage operation and low power consumption. As a next-generation memory having these characteristics, ReRAM (Resistance RAM) with an oxide resistance change element used for a storage element attracts attention.
FIG. 1 shows a typical oxide resistance change element 10. The oxide resistance change element 10 is structured to hold a metal oxide film 11 between an upper electrode 12 and a lower electrode 13. The metal oxide film 11 is made of, for example, transition metal oxides such as TiON and NiO. According to a structure disclosed by Hosoi Y. et al., “High Speed Unipolar Switching Resistance RAM (RPAM) Technology”, IEDM Technology Digest, 13 Dec. 2006, pp. 793-796, the metal oxide film 11 is made of TiON while the upper electrode 12 and the lower electrode 13 are made of TiN.
In the oxide resistance change element 10 having such a structure, an electric resistance in the metal oxide film 11 can be reversibly changed by several digits by applying a voltage pulse between the electrodes 12 and 13. More specifically, the oxide resistance change element 10 possibly has two states including a “low resistance state (i.e. metal state)” and a “high resistance state (i.e. insulator state)”. When a first write voltage is applied to the oxide resistance change element 10 in a low resistance state, the oxide resistance change element 10 is brought into transition to a high resistance state. In contrast, a second write voltage is applied to the oxide resistance change element 10 in a high resistance state, the oxide resistance change element 10 is brought into transition to a low resistance state. The oxide resistance change element 10 thus stores data in a nonvolatile manner by using an electric resistance change in the metal oxide film 11 which functions as a recording layer. For example, a low resistance state is made to correspond to data “1” and a high resistance state is made to correspond to data “0”.
FIG. 2 shows a structure of a write circuit with respect to the oxide resistance change element 10. A first write voltage source 101 generates a write voltage V0 for writing data “0”. Meanwhile, a second write voltage source 102 generates a write voltage V1 for writing data “1”. A selector 103 selects the write voltage V0 or V1 based on data to write. As a result, the write voltage V0 or V1 is applied to the oxide resistance change element 10, thereby data to write is written therein. Also, in FIG. 2, a current clamper 109 is connected in series between the second write voltage source 102 and the selector 103. In the aforementioned document, a series resistor is inserted.
FIG. 3 is a view for explaining conceptually data writing by the write circuit shown in FIG. 2. Each graph in FIG. 3 includes a lateral axis indicating a voltage applied to the oxide resistance change element 10 and a vertical axis indicating a current flowing into the oxide resistance change element 10. A broken line also indicates I-V characteristics in each of the data “1” state (i.e. low resistance state) and data “0” state (i.e. high resistance state). Since a resistance value in the data “0” state is larger than that in the data “1” state, when the same voltage is applied to the element in each state, a current flowing in the data “0” state is smaller than that in the data “1” state.
In writing data “0”, the write voltage V0 is applied. If the write voltage V0 is applied to the oxide resistance change element 10 in the data “1” state, the state is brought into transition to the data “0” state. A maximum value of a current flowing at this time is Ia. In transition of the state to the data “0” state (i.e. high resistance state), a current flowing therein is reduced.
In writing data “1”, the write voltage V1 is applied. The write voltage V1 is larger than the write voltage V0 (i.e. V1>V0). If the write voltage V1 is applied to the oxide resistance change element 10 in the data “0” state, the state is brought into transition to the data “1” state. In transition of the state to the data “1” state (i.e. low resistance state), a current flowing therein is increased. At this time, there is a danger that the write voltage V1, which is larger than the write voltage V0, may create an extremely large current flow to damage the oxide resistance change element 10. In order to prevent the danger, the current clamper 104 is connected in series between the second write voltage source 102 and the selector 103 as shown in FIG. 2. Owing to this current clamper 104, a current flowing in writing data “1” is suppressed to a maximum value Ib (i.e. Ib<Ia). Therefore, reciprocation of the state between the data “0” state and the data “1” state can also be prevented.
Note that a read current VR is applied in reading data. Based on a current value flowing in response to the read voltage VR, a resistance value of the oxide resistance change element 10, that is, data can be detected. The read voltage VR is established to be smaller than the write voltages V0 and V1 so as to prevent data writing in reading data.